What are the threats to our oceans?
Giant ships, using the latest equipment, can find schools of fish quickly and accurately. Modern fishing practices are incredibly wasteful. When fishers target particular species that they want to catch, they also catch other species accidentally. These non-target species are called bycatch and can include fish, marine mammals, seabirds or invertebrates such as crabs and shells.
Most modern fishing practices are incredibly wasteful. Every year, fishing nets kill up to 300,000 whales, dolphins and porpoises globally. Entanglement in lines and nets is the greatest threat to the survival of many species including some birds. As larger fish species decline in numbers, the next smaller fish species are targeted and so on.
More on bycatch
As fisheries in the northern hemisphere in particular have become depleted in stock more fish are being illegally caught around Africa and the Pacific depriving local inhabitants of major resources - food and income.
Habitats can also be altered by fishing. For example, bottom trawling, can destroy many fragile ecosystems.
Also see "So what's wrong with fish?". This an excellent detailed article covering a wide range of issues related to fishing. Highly recommended.